Ecosystems are divided by trophic levels, which categorizes every organism by class of what kind of consumer they are. 90% of the energy that is available for an organism is lost as it is transferred through each trophic level. Producers, such as the plants of an ecosystem, have available 100% of the energy they can harvest and use 90% of it, thus leaving only 10% of energy left for primary consumers. Primary consumers then leave only 1% of that original energy for secondary consumers. Then secondary consumers leave only .01% of the initial energy for tertiary consumers. It is called the rule of 10% in biology and may be easier to comprehend as a list.
Producers energy total: 100%
Primary consumers energy total: 10%
Secondary consumers energy total: 1%
Tertiary consumers energy total: .01%
And so on...